The UK’s democracy watchdog said in a statement: ‘The Electoral Commission has today announced it has opened an investigation to establish whether Momentum, a registered non-party campaigner at the 2017 UK Parliamentary General Election, breached campaign finance rules in relation to spending.’

The Electoral Commission said its probe would look into whether Momentum’s spending breached the limits for non-party campaigners in national elections.

Momentum backs Labour Party leader Jeremy Corbyn (Picture: Getty)

And it will also consider whether returns submitted by the group, founded in 2015 as a grassroots movement to support Mr Corbyn’s leadership of the Labour Party, accurately recorded donations and payments relating to the 2017 campaign.

The Commission’s director of political finance and regulation and legal counsel, Bob Posner, said: ‘Momentum are a high-profile active campaigning body. Questions over their compliance with the campaign finance rules at June’s general election risks causing harm to voters’ confidence in elections.

‘There is significant public interest in us investigating Momentum to establish the facts in this matter and whether there have been any offences.

‘Once complete, the Commission will decide whether any breaches have occurred and, if so, what further action may be appropriate, in line with its enforcement policy.’

‘Our election campaign was delivered on a low budget because it tapped into the energy and enthusiasm of tens of thousands of volunteers across the country.

‘We have a good working relationship with the Electoral Commission, and will fully comply with the investigation going forward.’

Electoral law imposes strict spending limits on non-party campaigners seeking to influence people to vote for one particular political party or any of its candidates.

Spending is limited to £31,980 in England, £3,540 in Scotland, £2,400 in Wales and £1,080 in Northern Ireland for the regulated period, which in this year’s case stretched for 12 months before the June 8 ballot.